Gonarch (lvl 36) -- Gonarchs are the final step to a headcrab's evolution, a Gonarch can gives birth to 3 Baby Headcrabs at once when attacking an enemy, but can also use ranged attacks aswell as sweep it's foes away if they get too close to it. A Gonarch should be a fairly hard encounter depending of the zone you are fighting it.

Gonome (lvl 12) -- Gonomes are ones of the most powerful type of zombies controlled by headcrabs, they are quite resistant, fast, can deal a lots of damage in close-combat and also have a slow predictable ranged attack. A gonome is fairly easy to kill depending of the zone you are fighting it.

Gargantua (lvl 70) -- Gargantuas are rare, powerful creatures, they absorb a crazy amount of damage, are fast, have a target-locking (albeit slow) ranged attack, and are the most dangerous opponent you can fight in a melee encountering.